You do not need a specialized charger to charge a bank of 3-4 AGM batteries. Assuming you have your batteries wired in parallel, you can hook the charger to one and it will charge the entire bank without any troubles.

I would split the banks for charging and use 2 promariner prosport 20's. Charging batteries in parallel gets more complicated the more you add. Also, you need to make sure they are wired in such a manner they get equipotential voltage to each. This means one corner of the parallel ladder gets the positive battery hookup and the opposite corner of the parallel ladder gets the negative hookup. Grant can probably do a good job of explaining this, he experienced a recent issue that required replacement of all 8 batteries in a bank because the positive and negative were pulled from the same battery versus + from the nearest and - from the farthest. Sounds complicated but it's not.

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To reiterate what mikeski said. I spoke with an electrical engineer (that owned a business working on yachts) about batteries and charging them he told Mr to set up my batteries as mikeski described or you will have some batteries not getting a full charge. Your charger will shut down after the first battery in the back indicates a full charge while there will likely be batteries down the line not fully charged.

The charging systems of yachts have nothing much in common with the charging systems of a towboat. In addition to independent port, starboard and gen banks, larger yachts frequently have a major dedicated thruster bank on their own charger but without individual battery separation.
Here is the way I look at it.
If the paralleled batteries are tightly packed, connected with heavy enough gauge cables, the same brand, age, size and even production run, then I see no need to break up the bank into individual batteries for shore charging and maintenance. After all, you are not discharging them independently when used or servicing them from the alternator independently.
There are exceptions.
A series bank, like a large boat 24 volt yacht system that supplies the bridge or trolliing motor bank will allow the batteries to be profiled and conditioned independently with the jumpers in place as long as the circuit is not closed.
A few chargers can provide more current capacity when all banks are used even though when connected to paralleled batteries they cannot read and react independently.
With a large number of batteries in a paralled bank it makes sense to remove the jumpers periodically, charge the batteries independently and measure the batteries independently after several days. If you catch a bad battery with accelerated self-discharge early enough in the bank's lifespan you might be able to replace just that battery without changing the entire bank. It's less than ideal but less expensive in the short run. When the entire bank is exhausted you will need to replace the newest battery. If the problem has existed for too long on batteries with too much age then you have to replace the entire bank. So again, while charging each battery with its own charger bank, you are still using (discharging/recharging) the batteries in parallel when on the water and unless the jumpers are broken in storage the separate charger banks cannot read individual batteries. That is a lot of solenoids or manual disconnects if you want total isolation in storage. Not sure how practical all that is.

He knew exactly what my set up was/is but as a reminder I do run two 8D's with 4/0 running between them.

The way he explained it made a lot of sense and has worked great for me.

Hate,
No doubt. Those 8Ds are big boys. Guessing each is about four times the A/H capacity of a group 24. If I had a 3-bank charger, I would also run a separate bank to each 8D just to ensure I left no amperage capacity on the table. The charger cannot profile and condition the two paralleled batteries independently until you totally isolate them. It sees the two batteries as one.